Very few big movies get made in Hollywood without multiple screenwriters. Maybe the studio buys an intriguing screenplay written by a newcomer and hands it off to a more established name for a rewrite. Maybe a director signs onto the project brings along a longtime script collaborator. Maybe the star demands frequent dialogue punch-ups on the set. In the end, the Writer’s Guild arbitration process sifts through the wreckage of infinite drafts to provide the illusion of order in the credits. Who gets a “Story By”? Who gets a “Screenplay By”? Meanwhile, Hollywood’s finest script doctors toil in well-paid obscurity. (History will record that John August probably wrote one line of dialogue in every movie made in the 21st century.)

World War Z is only an exception because the rewriting process has been out in the open. Way back in 2008, Ain’t It Cool rhapsodized about a draft of the screenplay by Babylon 5 creator J. Michael Straczynski; the script was abandoned in favor of a new take by Matthew Michael Carnahan, best known for political thrillers like The Kingdom and State of Play. Last year, Lost co-creator Damon Lindelof very publicly joined the troubled production with the stated purpose of crafting a new ending; Lindelof brought along Drew Goddard, a co-writer of Cabin inthe Woods who also worked on Lost and Buffy the Vampire Slayer. (If you’re keeping track, that means World War Z shares a DNA with three of the greatest genre shows in modern TV history.)

The film in theaters credits all of those writers: Straczynski and Carnahan share the “Story By,” while Carnahan shares the “Screenplay By” with the Goddard/Lindelof Ampersand team. Laura M. Holson’s fascinating Vanity Fair profile of the production also claims that Christopher McQuarrie (The Usual Suspects, Jack Reacher) played a role in the salvage effort. The most interesting thing about World War Z is that you can apparently identify the specific moment when the rewrites kick in. Brad Pitt’s character spends the middle part of the movie in Jerusalem, racing away from zombies and hopping on a plane. Originally, that plane was going to take him to Russia, where there would be a gigantic zombie action scene featuring Pitt hacking his way through mountains of zombies. This is apparently the part of the film shot in Budapest which, as reported by EW’s Geoff Boucher in his World War Z cover story, mostly fell away to the cutting room floor.

Lindelof himself tells Vanity Fair that, when he joined the project, he envisioned two roads to saving the production. One road would have been a soft-touch rewrite; “Road Two, which I think is the long-shot road, is that everything changes after Brad leaves Israel.” The studio opted for the second option, which is why the final act of World War Z feels radically different from the rest of the movie.

The film begins with an epic scope, with gigantic action sequences that race throughout major cities; but in the finished product, Pitt boards a plane to Cardiff, and the final sequence feels more like a classic zombie horror movie, with Pitt and an essentially all-new supporting cast walking quietly through darkened corridors. (You see hundreds of people die in the first 3/4 of the movie; after the plane crash, the living/undead body count doesn’t even come close to an episode of The Walking Dead.)

To me, though, the funny thing is that the radical change actually seems to work. This might just be because World War Z is a film constructed out of episodes, with roughly the same three-act set-up. Act One: Brad Pitt goes to a city. Act Two: Zombies attack. Act Three: Brad Pitt runs from attacking zombies. The original Russian-set final act sounds like an extension of the Jerusalem scene, with hordes of digital zombies who all move like wildebeests. But the Wales sequence raises the stakes by lowering them: Brad Pitt and his friends need to get somewhere to get something; in order to do that, they need to avoid the zombies. It’s the kind of simple plot that would’ve made for a great episode of Lost, and it brings an element of humanity to a movie that previously traded in gigantic disaster setpieces.

However, that isn’t quite the ending of the film – and this is where World War Z shows its hand a little too much. The filmmakers told EW that they envisioned World War Z as the beginning of a possible trilogy. Originally, the ending would not have reunited Pitt’s character with his family; Holson describes it as “a cliff-hanger that could be played out over future movies.” Neither the original ending nor the book World War Z features any indication of a major medical breakthrough in the fight against zombies – a breakthrough that, in the released film, provides the whole raison d’etre for the final-act trip to Wales. It’s established that zombies don’t attack people suffering from terminal illness; in a nutshell, zombies can smell cancer, and don’t want to eat it. The vaccine works by, apparently, infecting carriers with some strain of a terminal illness. This effectively makes people invisible to attacking zombies. The closing montage establishes that the armies of the world are all taking the vaccine; we also see it being air-dropped around the world. This would seem to mark the end of zombie-human hostilities. Like, even if many people, or entire countries, turn zombie before getting vaccinated, theoretically a tiny squad of zombie-proof soldiers could clear out an entire Undead City.

But the ending insists that it’s not an ending. Literally; Pitt’s final voiceover begins with the star insisting, “This isn’t the end. Not even close.” Despite the rough buzz, World War Z seems poised for a good run at the box office; even if the movie doesn’t entirely recoup its massive budget, it’s not outside the realm of possibility that Paramount would greenlight a slightly-cheaper sequel. (Keep in mind: This is the studio that, thanks to the modern franchise economy, is working on a sequel to Hansel and Gretel: Witch Hunters.) The question is: What would a sequel to World War Z look like? Would the zombies somehow mutate, or develop a resistance to the vaccine? Or maybe the film would focus more on recreating the earth after the apocalypse, with different factions assuming control of the ruins of the old globalized world? Will Mireille Enos ever smile? And, more to the point, would you want to see a sequel?